During August 1989, extended twilight and nighttime measurements with the European incoherent scatter (EISCAT) UHF radar were performed under PCA conditions. This provided an excellent data quality in the altitude region of 70 to 90 km throughout the three nights of August 12 to 15, 1989. A sophisticated control program allowed the measurement of the spectral width in the altitude region mentioned. In general, the measured spectral width deviates significantly from model values based on temperatures measured simultaneously by Na lidar combined with CIRA 88 temperature and density values. The observed spectra are up to 2 or 3 times narrower. In our observations the deviation tends to increase with increasing altitude. We also find that earlier spectral width measurements published by other workers are often narrower than current D region theory predicts. The possible reasons for this phenomenon are discussed.